The present invention relates generally to copper-bismuth alloys and, more particularly, to virtually lead-free copper base casting alloys which can be substituted for conventional leaded brasses and bronzes in plumbing fixtures and other applications.
Lead, as part of traditional copper base alloys, provides two major benefits, namely, improved pressure tightness and easy machinability. Because the solubility of lead in the copper matrix upon freezing at room temperature is 50 parts per million (0.005%), it has a tendency to segregate into areas which freeze last. As a result, it will fill in any voids which may exist in the casting thereby improving pressure tightness.
Also, in copper base alloys, the distribution of lead is nonuniform in nature. This segregation of lead aids the machinability index because the tool will touch the lead-rich surfaces in the casting thereby making it easier to form small chips with ease. The presence of lead in copper base castings also makes them much easier to polish which is highly desirable as many plumbing fixtures are plated with chrome.
Nevertheless, despite the favorable casting characteristics described above, the presence of lead in castings to which people may be exposed and which are also presently utilized in a variety of manufacturing processes has created far more serious problems in the areas of health as it relates to ambient air, potable water, and the soil system. These problems are currently and forthrightly being addressed by the Occupational, Safety and Health Administration (OSHA), the Environmental Protection Agency (EPA), and both Houses of Congress.
As a consequence, OSHA is requiring all foundries that employ more than 20 people to reduce their plant ambient air levels to 50 .mu.g of lead per cubic meter of air from the present standard of 200 .mu.g by July 1996. This will cause millions of dollars to be spent on unproductive equipment at the affected businesses in the coming years. Currently, the EPA is moving toward reducing the lead leaching standard in drinking water from 50 .mu.g/L, its present level, all the way down to possibly as low as 5 .mu.g/L. Both Houses of Congress are considering a variety of measures dealing with this issue.
While the affected industries have made substantial efforts to develop a lead-free alloy, currently no such alloy is being used which is technologically feasible or economically viable in the ways discussed below. To be commercially viable, this alloy must possess acceptable castability, machinability, solderability, plateability, and resistance to corrosion characteristics. It would also be highly beneficial to all foundries if the desirable lead-free alloy could also be cast in a similar fashion to the present leaded alloys thereby eliminating the need for worker training or the purchase of new equipment. Finally, it would be highly desirable if the scrap generated from the production and use of these lead-free castings would not contaminate the scrap of the presently used leaded copper base alloys, if mixed. This would have tremendous appeal to the recycling industry--a highly beneficial and growing industry in the U.S.
One approach that has been taken to provide lead-free copper alloys is to substitute bismuth for the lead in the alloy composition. Bismuth, which is adjacent to lead in the Periodic Table, is non-toxic. It is virtually insoluble in the solid state and precipitates as pure globules during freezing in a copper base alloy. When alloyed with copper, bismuth produces a course grain size that promotes shrinkage porosity. For many years it has been recognized that bismuth is brittle as cast in copper base alloys. Nevertheless, some success with lead-free or substantially lead-free bismuth-containing copper alloys has been reported in the patent literature.
U.S. Pat. No. 4,879,094 to Rushton discloses a cast copper alloy which contains 1.5 to 7% bismuth, 5 to 15% zinc, 1 to 12% tin and the balance essentially copper.
Japanese Published Applications 57-73149 and 57-73150 to Hitachi disclose copper alloys containing bismuth which are characterized by additions of graphite and titanium or manganese. Chromium, silicon, or mischmetal may be added to the alloy.
U.S. Pat. No. 5,167,726 to AT&T Bell Laboratories discloses a wrought copper alloy containing bismuth and phosphorous, tin or indium.
U.S. Pat. No. 5,137,685 discloses a copper alloy in which the lead content is reduced by the addition of bismuth. The alloy nominally contains 30 to 58% zinc. To improve its machinability, a sulfide, telluride, or selenide may be added to the alloy or, to enhance the formation of sulfides, tellurides and selenides, an element which combines with them such as zirconium, manganese, magnesium, iron, nickel or mischmetal may be added.
U.S. Pat. No. 4,929,423 discloses a lead-free solder containing 0.08 to 20% bismuth, 0.02 to 1.5% copper, 0.01 to 1.5% silver, 0 to 0.1% phosphorous, and 0 to 20% mischmetal and the balance tin.
The cost of alloys containing large quantities of bismuth is another concern because bismuth is much more expensive than lead. Questions arise concerning the cost compatibility of bismuth containing alloys as substitutes for leaded alloys. If bismuth-containing lead-free alloys are too expensive, industry may adopt less satisfactory substitutes such as plastic. While there have been numerous attempts to provide low lead or lead-free copper base alloys, to date, none have proven to be commercially successful.